1. Field of the Invention
This invention relates to techniques for growing nanotubes from carbon and other materials, and more particularly to a method of epitaxial growth of a nanotube precursor with specified chirality and cloning of the precursor to grow a specified chiral nanotube.
2. Description of the Related Art
Carbon nanotubes (CNTs) have stimulated a great deal of interest in the micro/nano-electronic and other industries because of their unique properties including tensile strengths above 35 GPa, elastic modulus reaching 1 TPa, higher thermal conductivity than diamond, ability to carry 1000× the current of copper, densities below 1.3 g/cm3 and high chemical, thermal and radiation stability. CNTs have great promise for devices such as field effect transistors, field emission displays, single electron transistors in the microelectronic industry, and uses in other industries. Commercialization of CNTs will depend in large part on the ability to grow and network CNTs on a large cost-effective scale without compromising these properties.
A CNT is a hollow cylindrical shaped carbon molecule. The cylindrical structure is built from a hexagonal lattice of sp2 bonded carbon atoms with no dangling bonds. The properties of single-walled nanotubes (SWNTs) are determined by the orientation of the rolled graphene structure in which the carbon atoms are arranged to form the cylinder. Multi-walled nanotubes (MWNTs) are made of concentric cylinders around a common central hollow. The orientation of the hexagonal lattice can exhibit different ‘chirality’ e.g. armchair, zig-zag, and chiral as specified by their n,m type. The different chiralities exhibit different electrical and thermal conductivities and different growth rates.
CNTs are commonly grown using several techniques such as arc discharge, laser ablation and chemical vapour deposition (CVD). In CVD the growth of a CNT is determined by the presence of a catalyst, usually a transition metal such as Fe, Co or Ni, which causes the catalytic dehydrogenation of hydrocarbons from a carbon-containing growth gas, typically a hydrocarbon CxHy such as Ethylene (C2H4), Methane (CH4), Ethanol (C2H5OH), or Acetylene (C2H2) or possibly a non-hydrocarbon such as carbon-monoxide (CO), and consequently the formation of a CNT which also matches the symmetry and lattice constants of the catalyst. CVD is relatively easy to scale up and can be integrated with conventional microelectronic fabrication, which favors commercialization.
The way in which nanotubes are formed at the atomic scale is not precisely known. The detailed growth mechanism is still a subject of scientific debate, and more than one mechanism might be operative during the formation of CNTs. A catalyst is deposited on a support such as silicon, zeolite, quartz, or inconel. At elevated temperatures, exposure to a carbon containing gas causes the catalyst to take in carbon, on either the surfaces, into the bulk, or both. This thermal diffusion process of neutral carbon atoms occurs at energies of a few electronvolts (eV). A precursor to the formation of nanotubes and fullerenes, C2, is formed on the surface of the catalyst. From this precursor, a rodlike carbon is formed rapidly, followed by a slow graphitization of its wall. The CNT can form either by ‘extrusion’ (also know as ‘base growth’ or ‘root growth’) in which the CNT grows upwards from the catalyst that remains attached to the support, or the catalytic particles can detach from the substrate and move at the head of the growing nanotube, labelled ‘tip-growth’. Depending on the size of the catalyst particle either SWNT or MWNT are grown. A typical catalyst may contain an alloy of Fe, Co or Ni atoms having a total diameter of 1 to 100 nm (on the order of 1,000 atoms for 1 nm diameter of catalyst). The diameter of the CNT also depends on the diameter of the catalyst but cannot be precisely controlled. Furthermore, the carbon nanotubes will exhibit different chiralities somewhat randomly across an array.
Conventional nanotube growth techniques produce arrays of hundreds of thousands to tens of millions of nanotubes in which the chirality of the nanotubes varies randomly throughout the array. In many applications, either a uniform chirality whatever it may be or a particular chirality is required or at least desired. Currently, this requires the use of an atomic force microscope and many hours of labor to sift through the free nanotubes once they are harvested to extract those of a desired chirality. More recently chemical methods have been developed to sort based on chirality but these methods are expensive, time consuming and also involve dangerous poisons.
Richard E. Smalley et al. “Single Wall Carbon Nanotube Amplification: En Route to a Type-Specific Growth Mechanism” J. Am Chem Society Nov. 15, 2006, 128, 15824-15829 describes a technique to mass produce any specific n,m type of SWNT from a small sample of the same material. The ultimate protocol would involve taking a single n,m-type nanotube sample, cutting the nanotubes in that sample into many short nanotubes, using each of those short nanotubes as a template for growing much longer nanotubes of the same type, and then repeating the process.